The present invention relates to a device for the exchange of heat between air flowing into and out of indoor premises, respectively called supply air and exhaust air, which device is composed of two heat exchangers each containing sets of passages that are separate but arranged to permit the mutual exchange of heat, one set of passages carrying a liquid that circulates, via lines, between the two exchangers, while the other set carries the respective air streams. This type of airconditioning exchanger is generally less efficient than the regenerative type, with a rotor to transfer heat between the two air streams as the latter pass through it. One of the advantages of a set-up with separate heat exchanger units is that while the regenerative exchanger requires air ducts for both streams of air to be routed to the exchanger, two separate exchangers can be installed wherever there is room for them in the building, e.g. in the cellar and the loft of a private house. The connecting lines for the circulating liquid can be of considerably smaller cross-section than the air duct of the regenerative exchanger, and are hence easy to route through the house. This is especially true in cases where it is desired to improve the heating economy of an existing building by installing airconditioning exchangers.
In winter there is an especially urgent need to reclaim as much as possible of the heat contained in outgoing air, and the main task of the invention is to ensure defrosting of the exhaust heat exchanger where, because of the loss of heat to the circulating liquid, the exhaust air often reaches a relative humidity of 100% and a temperature low enough to cause moisture to condense and form a deposit of ice on the walls of the air passages.